Geothermal energy has been a hot topic of discussion as humanity moves toward more diverse and less environmentally impactful energy production in the future. How important is geothermal going to be, and how do we make the most use of its potential?
First, a definition. “Geothermal” is literally heat from the earth. It is very hot below our feet – up to 6,000 degrees Celsius at Earth’s core (Figure 1).
Figure 1. Earth’s inner layers showing increasing heat with depth, until temperatures reach 6000 degrees Celsius in the inner core
From the surface, the temperature increases by about one degree Celsius for every 20 to 50 metres of depth – a rate called the geothermal gradient. Where the gradient is high, very hot rock can be found at relatively shallow depths – think of Iceland or Hawaii, where there is molten rock (magma) at shallow depths that escapes to the surface as lava from volcanoes. Less extreme examples are found in many western parts of Canada and the United States, where geothermal water comes to surface in geysers or hot springs (Figure 2).
Figure 2. Hot springs like this one near Nakusp, B.C. are found where warm geothermal waters come to surface.
Geothermal energy is regarded as a renewable, low-emissions energy source that we tap into by drilling wells to access hot water in permeable rocks. Once the wells are drilled and the facilities are built, power generation is practically emission-free, particularly if pumps and other equipment are run off electricity generated at the facility. Geothermal also has the tremendous advantage of producing energy continuously, not intermittently, as in the cases of wind and solar.
We can use geothermal energy in two ways. Many people think of using very hot geothermal waters to drive turbines creating electricity. That is important, but “direct heat” applications – which can use much cooler geothermal waters for greenhouses, space heating, and other applications – actually consume more geothermal energy.
Geothermal electricity
To generate geothermal electricity, we need a lot of very hot water (>120 degrees C) in a superheated state under elevated pressures at depth that will flash to steam when brought to the surface. Somewhat cooler waters can be used to heat an intermediate fluid that will boil at lower temperatures, but that’s a less efficient process. Places in the world with current or recent volcanism, and thus a lot of hot rock close to the surface, are ideal for geothermal electricity generation. Iceland, Japan, Kenya, Indonesia, New Zealand, and the western U.S. are all geothermal generation centres (Figure 3).
Figure 3. Nations producing large amounts of geothermal electricity, showing geothermal generation capacity in megawatts
While the U.S. is a geothermal electricity player, Canada does not generate any geothermal electricity today, as most of the country features low geothermal gradients. In the Rocky Mountains and westward, there are higher gradients, but waters are still generally too cool for power generation. Several innovators are trying to overcome the challenge of accessing sufficient hot water energy:
- Eavor Technologies Inc. started in Western Canada but is working internationally to develop their closed-loop geothermal power system using advanced oilfield horizontal drilling and geosteering techniques. They have drilled a demonstration facility in west-central Alberta.
- The U.S. Department of Energy Frontier Observatory for Research in Geothermal Energy (FORGE) in Utah is a dedicated geothermal test field where enhanced geothermal systems (EGS) technologies can be developed and tested.
- Global geothermal energy developments are highlighted at Think Geoenergy
Geothermal direct heat
Turning to direct heat use of geothermal, there are many applications – some new, and some older ideas getting a new look because of geothermal’s low-emissions, always-ready energy supply.
Hot water from geothermal wells can feed directly into district heating systems, where heat is collected at a central location and distributed to a network of buildings and infrastructure for space heating and hot water supply. The Capitol Mall geothermal project heats 92 buildings in Boise, Idaho, along with a swimming pool and sidewalks. Geothermal water at about 80C is pumped from wells up to 800 metres deep, circulated through the system, and re-injected underground once the useful heat has been extracted. District heating works best in densely populated centres, where there are many users close together, so that heat is not lost in transporting hot water over substantial distances.
Other direct-use geothermal applications include snow melting, fish farming, swimming pools, greenhouses, and food drying and canning. Geothermal is particularly useful in remote locations where it is difficult to bring in other energy sources in quantity, and where demand in concentrated – as in large-scale greenhouse operations.
Heat pumps are a well-established technology to heat and cool buildings that also make use of the earth’s heat capacity. However, these are not true geothermal applications because they do not actually extract heat from deep in the earth, but instead use relatively shallow zones (up to tens of metres) for heat storage. The earth at shallow depths is warmer than air temperatures in the winter, and cooler in the summer. A heat pump circulates fluid from a building into an array of pipes a few metres below ground to cool the air in the summer, and to heat it in the winter – thus saving on electricity for air conditioning, and electricity or fossil fuels for space heating.
Geothermal heat and power in the energy transition
There are several organizations providing information on geothermal potential in Canada (Canadian Geothermal Energy Association, or CanGEA) and internationally (Global Geothermal Alliance and Proceedings of the World Geothermal Conference).
Some promoters would have us believe that geothermal potential is unlimited, and can power the world once we build enough facilities – see the August 2021 TED talk by Jamie Beard.
But like every other energy source, geothermal has positive and negative attributes, and there are limitations to its potential.
Most important, like every other resource, readily available and affordable geothermal energy occurs only in certain places. Figure 4 shows that hot shallow rocks occur at the boundaries of Earth’s tectonic plates, which is why geothermal power generation is big in places such as Iceland, Japan, and Indonesia. Where the geothermal gradient is lower, we have to spend a lot more money to drill down to rocks that are hot enough.
Figure 4. Map of surface geothermal heat flow, highlighting very hot areas along tectonic plate boundaries
Regardless of the geothermal fluid temperature, we have to move large volumes of hot water to make a geothermal project economically viable. Geothermal wellbores are thus generally quite large – much larger than oil or gas wellbores, and consequently more expensive. The geothermal reservoir rock must be highly permeable to flow large fluid volumes rapidly. If it is not sufficiently permeable, we can induce fractures in the rock to create enhanced geothermal systems (being tested at FORGE) – a process much like hydraulic fracturing in oil and gas wells. Or, we can use a closed-loop technology such as Eavor’s, so that fluids to be heated are circulated within a self-contained, multi-well system in the geothermal reservoir, and never actually have to flow through the rock.
To sum up, geothermal energy will be a significant player as we progress to more diverse energy sources. It offers low-emission, reliable energy in places where there is plenty of heat at relatively shallow depths and appropriate reservoirs with a lot of hot water. Geoscience and engineering datasets and skills from the oil and gas industry are valuable in both finding good geothermal resources and in creating innovations to tap into those resources more efficiently and cost effectively.
Thanks Lee, much appreciated
Well written, Brad.